This spontaneous transfer of electrons from zinc to
copper is not useful because the free energy released
is dissipated as heat.
It is analogous to burning a spoonful of sugar with
a match instead of eating it and converting the free
energy of oxidation into useful muscle work.
If some means could be found to separate the removal
of electrons from zinc (oxidation)
from the donation of electrons to copper ions (reduction),
then the electrons might be made to do something useful
along the way.